This invention relates to a recording medium reproducible by utilizing near-field light and a near-field optical probe for reproducing information recorded on such a recording medium and, more particularly to a recording medium and near-field optical probe that enhances reproducing resolution for information recorded with density.
In recent years, remarkable development has been made in optical reproducing apparatuses (DVD players, etc.) for reproducing information on recording media by illuminating laser light. However, the information recording density has reached a limitation because of a presence of a diffraction limit of laser light. In an attempt to break through such diffraction limit, a proposal has been made on a near-field light reproducing apparatus using an optical head provided with a microscopic aperture having a diameter of less than a wavelength of laser light to be utilized in reproducing so that near-field light (including both near field and far field) produced at the microscopic aperture or on a surface of the recording medium can be utilized, thereby increasing reproducible information recording density.
Conventionally, the near-field microscopes using a probe (hereinafter referred to as a near-field optical probe) having a microscopic aperture as mentioned above as an apparatus utilizing near-field light have been utilized for observing sample microscopic surface textures. As one of schemes utilizing near-field light in the near-field microscopes, there is a scheme that the near-field optical probe microscopic aperture and the sample surface are approached in distance to nearly a diameter of the near-field optical probe microscopic aperture so that near-field light can be produced at the microscopic aperture by introducing propagation light through the near-field optical probe and toward the near-field optical probe microscopic aperture (illumination mode). In this case, scattering light caused by the interaction between the produced near-field light and the sample surface involving an intensity and phase reflecting a sample surface microscopic texture is detected by a scattering light detection system. Thus, high resolution of observation is made feasible that could not be achieved by the conventional optical microscopes.
There is another scheme of the near-field microscopes utilizing near-field light that propagation light is illuminated toward a sample to localize near-field light over the sample surface wherein a near-field optical probe microscopic aperture is approached to the sample surface to nearly a diameter of the near-field optical probe microscopic aperture (collection mode). In this case, scattering light cause by the interaction between the localized near-field light and the near-field optical probe microscopic aperture involving an intensity and phase reflecting a sample surface microscopic texture is guided to a scattering light detection system through the near-field optical probe microscopic aperture, thus achieving observation with high resolution.
As a near-field microscope, Japanese Patent Laid-open No. 174542/1995, for example, has been proposed disclosing a scanning near-field atomic force microscope. This scanning near-field atomic force microscope adopts as near-field optical probe an optical waveguide sharpened at a tip to perform probe access control and scanning control for the atomic force microscope (AFM) thereby enabling observation of sample surface topology and optical characteristics. FIG. 11 is a block diagram showing a schematic configuration of the scanning near-field atomic force microscope.
In FIG. 11, a scanning near-field atomic force microscope 80 has, above a probe 89, a laser light source 83, a focus lens 84, a mirror 85 and a photoelectric conversion element 86 vertically divided into two. The light emitted from the laser light source 83 is collected by the focus lens 84 onto a probe top surface 82 so that the light reflected thereon is guided to the photoelectric conversion element 86 via the mirror 85. Meanwhile, the light emitted from a light source 94 for light information measurement is illuminated through a collimate lens 95 to a backside of a recording medium 81 over a prism 92 having a slant face treated for total reflection. Then, the light is guided to the other end of the probe 89 (not-sharpened base) that is proximate to the recording medium 81 and introduced to the photoelectric conversion element 87.
The prism 92 and recording medium 81 are set up on a rough movement mechanism 97 and fine movement mechanism 96 movable in XYZ directions. The signal detected by the photoelectric conversion element 86 is sent to a servo mechanism 93. Based on the signal, the servo mechanism 93 controls the rough movement mechanism 97 and fine movement mechanism 96 so that the deflection on the probe 89 cannot exceed a prescribed value when approaching of the probe 89 to the recording medium 81 or reading out data. The servo mechanism 93 is connected with a computer 99 to control operation of the fine movement mechanism 96 in a planar directions and receive information about the recording medium from a control signal of the servo mechanism 93. Meanwhile, when applying modulation to the light of the light source 94 or providing vibration by a vibration mechanism 88 to between the probe 89 and the recording medium 81, the signal obtained in the photoelectric conversion element 87 is connected to an analog input interface of the computer 99 via a lock-in amplifier 98 to detect optical information in synchronism with planar action of the fine movement mechanism 96. When no modulation or the like is applied to the light source 94, the signal obtained in the photoelectric conversion element 87 is directly connected to the analog input interface of the computer 99 without being passed through the lock-in amplifier 98.
The above near-field optical information reproducing apparatus utilizes the near-field microscope technology and observation scheme, and can reproduce information densely recorded on a recording medium by utilizing near-field light.
However, where the recording medium is increased in recording density by arranging data marks as information units in a close relationship, when conducting reproducing with the recording medium there encounters difficulty for the near-field optical probe used in the conventional near-field optical information reproducing apparatus to individually recognize and detect adjacent ones of the data marks. This problem is explained hereinbelow on an example of a near-field optical probe of a near-field optical information reproducing apparatus for information reproducing on the collection mode. FIG. 12 shows a recording medium 100 arranged with data marks 101 to produce near-field light. Incidentally, FIG. 12 shows one part of the recording medium 100 wherein the dotted circle 102 signifies a position that a data mark is possible to provide.
In FIG. 12 the data marks 101 are different in optical transmittance or refractive index, for example, from a base member 103 of the recording medium 100. The difference in optical property enables recognition of the presence or absence of a data mark 101. That is, in the data mark 101 the near-field light produced on a surface of the recording medium 100 is different in intensity or the like from that of the base material 103, which realizes to reproduce information configured by the data mark 101. Here, the near-field light on the surface of the recording medium 100 is produced by illuminating incident light, such as laser light, at a backside (surface not having data marks) of the recording medium 100 under a condition of total reflection. Incidentally, recording onto the data mark 101 is possible to realize by a phase change recording method or the like in the currently-marketed rewritable recording mediums.
FIG. 13 shows a relationship between a sectional view of the recording medium 100 taken on line D-Dxe2x80x2 in FIG. 12 and near-field light produced by the data marks 101. Meanwhile, in FIG. 13, a near-field optical probe 110 is arranged above the recording medium 100. The near-field optical probe 110 moves in a rightward direction in the figure as scanning directions to sequentially detect near-field light produced through the data marks 101 of the recording medium 100. For example, provided that in FIG. 13 the regions recorded with a data mark 101 (101a, 101b, 101c) are taken as xe2x80x9c1xe2x80x9d while those not recorded with a data mark 101 is as xe2x80x9c0xe2x80x9d, a signal will be reproduced as xe2x80x9c01101xe2x80x9d from left of the figure.
Accordingly, the amplitude of near-field light in positions fully close to the corresponding data marks 101a, 101b, 101c to xe2x80x9c1xe2x80x9d can be expressed rectangular, in an ideal case, as represented in a near-field light amplitude distribution of FIG. 13 (on medium surface). With respect to this, a near-field light amplitude distribution (at microscopic opening) of FIG. 13 shows a near-field light amplitude distribution of near-field light reaching a microscopic aperture 111 of the near-field optical probe 110, i.e. at a position that a given distance is provided between the data mark 101 and the near-field optical probe 110. Each data mark 101 has a spread smoothly attenuating left and right with a maximum point given on a center axis of the data mark.
Meanwhile, a near-field light intensity distribution (at microscopic aperture) of FIG. 13 illustrates a near-field light intensity distribution offered by the above near-field light amplitude distribution (at microscopic aperture). As is shown, near-field light produced through the adjacent data marks 101a and 101b at a position reaching the microscopic aperture 111 of the near-field optical probe 110 overlaps at respective foots of near-field light amplitude. This results in an obscured boundary between near-field light produced through the data mark 101a and near-field light produced through the data mark 101b, thus lowering resolution in reproducing. Thus, the data marks are difficult to separately recognize in the microscopic aperture 111 position of the near-field optical probe 110.
The near-field optical information reproducing apparatus is to ultimately detect a data mark 101 or reproduce information by guiding, into a near-field optical probe, scattering light (propagation light) obtained by scattering near-field light reaching the microscopic aperture 111 of the near-field optical probe 110. Consequently, the problem with data mark separation is not negligible. This problem might be avoided by providing full spacing between the data marks 101. This however decreases recording density on the recording medium, impairing high-density recording medium reproducing as a merit of near-field optical information reproducing apparatus.
Meanwhile, in illumination-mode information reproducing, in order to separately recognize individual data marks densely arranged on a recording medium, it is possible to decrease a localization range of near-field light caused at the microscopic aperture by reducing the size of the microscopic aperture of the near-field optical probe. However, a high level technology is required to make smaller microscopic aperture. There encounters a problem that a decreased localized range of near-field light is also decreased in intensity and hence difficult to detect.
It is an object of the present invention to provide a recording medium and near-field optical probe which can achieve high density information recording and enhance resolution in reproducing.
In order to achieve the above object, a first recording medium according to the present invention is a recording medium formed on a recording medium surface with information reproducible by utilizing near field light, comprising: a medium base member transparent for a wavelength of illumination light (incident light) to be illuminated to produce the near field light; a phase shifter arrangement layer arranged alternately, in a direction parallel with a surface of the medium base member and in a reproducing direction of the information, with transmission regions transparent for the wavelength of the illumination light and phase shifter regions transparent for the wavelength of the illumination light to cause a shift of phase in the illumination light by 180 degrees; and a data mark arrangement layer formed on the phase shifter arrangement layer and having data marks as units of the information respectively corresponding one by one to the transmission regions and the phase shifter regions.
According to this invention, on a base member (medium base member) having a sufficient transmittance for a wavelength of incident light to be illuminated at the backside of the recording medium where a near field light information reproducing apparatus adopts a collection mode as a reproducing scheme, a phase shift arrangement layer is provided which is formed alternately with a phase shifter region transparent for the wavelength of the incident light to be illuminated at the backside of the recording medium under a condition of total reflection to cause a 180-degree shift of phase in the incident light and a transmission region transparent for the wavelength of the incident light. Furthermore, a data mark arrangement layer is formed that a row of data marks can be arranged in positions on the phase shifter regions and transmission regions. It is therefore possible to cause cancellation between spread of near field light caused on the recording medium due to incident light transmitted through the phase shifter regions and spread of near field light caused on the recording medium due to incident light transmitted through the transmission regions.
Also, in a second recording medium of the invention, in the first recording medium the data mark arrangement layer has a shade film formed on a surface thereof in regions except for those having the data marks, to shade from the illumination light.
According to the invention, the shade film is coated on the surface of the data mark arrangement layer in areas except for the data marks and those where data marks are possible to provide. Accordingly, near-field light only is obtained on the surface of the recording medium for illumination light incidence under other conditions than total reflection.
Also, a third recording medium according to the present invention is a recording medium formed on a recording medium surface with information reproducible by utilizing near field light, comprising: a medium base member transparent for a wavelength of illumination light to be illuminated to produce the near field light; a phase shifter translucent for the wavelength of the illumination light to cause a shift of phase in the illumination light by 180 degrees; wherein the phase shifter is provided on a surface of the medium base member in areas except those for producing the near field light and having data marks as units of the information.
According to this invention, on a base member (medium base member) having a sufficient transmittance for a wavelength of illumination light (laser light or the like) to be illuminated at the backside of the recording medium where a near field light information reproducing apparatus adopts a collection mode as a reproducing scheme, a phase shifter is coated, which is translucent for the wavelength of illumination light to be illuminated at the backside of the recording medium to cause a 180-degree shift of phase in the laser light wherein an opening not coated with the phase shifter is arranged as a data mark as information unit. It is therefore possible to cancel between spread at data mark edges of near field light caused through the data mark and spread at phase shifter edges of near field light.
Also, a first near field optical probe according to the present invention is a near field optical probe having a microscopic aperture for producing near field light, comprising: a planar substrate formed with a through-hole; and a phase shifter translucent for a wavelength of illumination light to be illuminated for producing the near field light to cause a shift of phase in the illumination light by 180 degrees; wherein the phase shifter is provided in a manner closing one opening of the through-hole to have the microscopic aperture.
According to this invention, the phase shifter is formed at the opening of the through-hole of the near field optical probe which is translucent for a wavelength of laser light (illumination light) to be introduced to the through-hole to cause a 180-degree shift of phase in the laser light. Furthermore, the microscopic aperture is formed in the phase shifter to have an appropriate diameter for producing near field light. Accordingly, in the case that the reproducing scheme to be adopted in a near field light information reproducing apparatus is taken an illumination mode, cancellation is made between spread in near field light produced through the microscopic aperture and spread in propagation light transmitted through the phase shifter, at their respective edges.
Also, a second near field optical probe according to the present invention is a near field optical probe having a microscopic aperture for producing near field light, comprising: a planar substrate formed with a through-hole; a shade film opaque for a wavelength of illumination light to be illuminated for producing the near field light; and a phase shifter transparent for the wavelength of the illumination light to be illuminated for producing the near field light to cause a shift of phase in the illumination light by 180 degrees; wherein the shade film is provided in a manner closing one opening of the through-hole to have a first microscopic aperture for producing the near field light; and the phase shifter being provided in a manner closing the first microscopic aperture to have a second microscopic aperture smaller than said first microscopic aperture.
According to this invention, the first microscopic aperture for producing near field light is formed by locally coating the shade film. The phase shifter is formed in a manner closing the first microscopic aperture, which is transparent for a wavelength of laser light (illumination light) to be introduced to the through-hole to cause a 180-degree shift of phase in the laser light. The second microscopic aperture is provided in the phase shifter. Accordingly, in the case that the reproducing scheme to be adopted in a near field light information reproducing apparatus is taken an illumination mode, cancellation is made between spread in near field light produced through the first microscopic aperture and spread in second near field light transmitted through the phase shifter, at their respective edges. Thus, it is possible to obtain near field light with sharpness.